Background:  HAART therapy containing nucleoside analogs (NRTI) has associated mitochondrial toxicity that limits therapy. NRTI intracellular processing to active moieties is critical to therapeutic and toxic events. NRTI intracellular processing was examined in a transgenic mouse system to help understand mechanisms of mitochondrial toxicity from NRTI.

Methods:  Transgenic mouse cardiac targeted overexpression of the DNC gene was accomplished using the a-myosin heavy chain promoter. Two transgenic lines were created and exhibited normal survival, fertility and transgenic expression. Northern analysis of tissues verified substantial overexpression of DNC in heart without changes in other tissues. The cardiac phenotype was examined with and without administration of HAART containing NRTI. Echocardiograms (ECHO) and magnetic resonance imaging (MRI) of the heart determined left ventricle mass and electrocardiograms (ECG) defined cardiac rhythm disturbances. Mitochondrial polypeptide abundance was determined using Western blot analysis of complex I polypeptides. mtDNA mutations were defined by DNA sequencing. Plasma lactate was quantified as an index of mitochondrial dysfunction. Mitochondrial ultrastructural changes (TEM) were examined  in hearts from TGs and compared to hearts of wild type (FVB/n) littermates.

Results:  DNC transgenic mice (³ 8 to 12 weeks old) exhibited normal cardiac mass on ECHO but abnormal cardiac mitochondrial cristae and depleted complex I proteins. HAART treatment (zidovudint [AZT]/lamivudine [3TC]/indinavir [IDV] or stavudine [d4T/3TC/IDV; 35 days; human therapeutic doses) each increased left ventricular mass in transgenic mice on ECHO and MRI. Increased plasma lactate was present in transgenic cohorts that received AZT/3TC/IDV. Mitochondrial ultrastructural changes of mitochondrial myopathy were found with HAART, but mtDNA mutations were absent.

Conclusions:  DNC plays a role in intracellular pharmacological distribution of thymidine NRTI and affects mitochondrial toxicity from HAART. Cardiac overexpression of DNC yielded mitochondrial cristae structural abnormalities and mitochondrial polypeptide depletion without cardiac physiological changes. Addition of HAART resulted in profound mitochondrial destruction and dysfunction. Data suggest that NRTI intracellular processing is a critical event in NRTI mitochondrial toxicity in patients.

Keywords: mitochondria; cardiomyopathy; toxicity